Date: Mon, 6 Dec 2010 12:29:46 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
To: Matt Mackall <mpm@selenic.com>
cc: Tejun Heo <tj@kernel.org>, akpm@linux-foundation.org,
        Richard Kennedy <richard@rsk.demon.co.uk>,
        Pekka Enberg <penberg@cs.helsinki.fi>, linux-kernel@vger.kernel.org,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [cpuops inc_return V1 8/9] random: Use this_cpu_inc_return
In-Reply-To: <1291659102.3065.2212.camel@calx>
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References: <20101206173958.685460926@linux.com>  <20101206174017.684979312@linux.com> <1291659102.3065.2212.camel@calx>
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On Mon, 6 Dec 2010, Matt Mackall wrote:

> On Mon, 2010-12-06 at 11:40 -0600, Christoph Lameter wrote:
> > plain text document attachment (cpuops_inc_return_random)
> > __this_cpu_inc can create a single instruction to do the same as
> > __get_cpu_var()++.
>
> > -	    (__get_cpu_var(trickle_count)++ & 0xfff))
> > +	    ((__this_cpu_inc_return(trickle_count) - 1) & 0xfff))
>
> I see you've added a "- 1" to mimic the post-increment ++ semantic.
>
> The goal of this code to add only 1 sample in every 4096, but we don't
> really care which, so this semantic isn't actually important here.

-1 also removes the + 1 that the inc_return() semantics require because we
use xadd on x86. -1 will generate more compact code.

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